1. Field of the Invention
The present invention relates to a method for forming a film, a system for forming a film, a semiconductor device, and a fabrication method thereof.
2. Related Background Art
Thermal treatment systems being one of semiconductor equipment have been used heretofore as systems for forming a thin film of oxide on a substrate to be treated, such as a semiconductor substrate or the like. Such thermal treatment systems include, for example, systems for oxidizing the surface of the substrate by heating the substrate while supplying oxygen dried under almost ordinary pressure, to the substrate of semiconductor wafer supported on a substrate support member in a chamber (dry oxidation such as RTO; Rapid Thermal Oxidation, or the like), or systems for oxidizing the surface of the substrate by heating the substrate in an oxidizing atmosphere containing water generated by preliminary combustion of oxygen and hydrogen (wet oxidation by external combustion method). There are also attempts to employ a method of oxidizing the surface by exposing the substrate to radical oxygen (or oxygen radicals) generated by ultraviolet irradiation or the like.
Incidentally, with increase in integration density of such semiconductor devices as VLSI devices, there is the recently increasing desire of further thinning the film while maintaining reliability, for the devices with a thin film of gate oxide or the like. In fabrication of such semiconductor devices, there is always the earnest desire for further enhancement of productivity.
Against such needs, the film formation by conventional dry oxidation such as RTO was superior in repeatability and uniformity of thickness of the film formed, but there was such a tendency that film-forming rates (deposition rates) were not always adequately large. On the other hand, the film formation by wet oxidation in the external combustion method failed to provide adequate controllability of film-forming reaction because of the use of external combustion and thus involved the risk of degrading the repeatability and uniformity of thickness. In addition, it required a torch unit for externally burning oxygen and hydrogen. Meanwhile, the film formation by radical oxygen had the problem that it was necessary to use a unit such as an ultraviolet irradiating unit, a plasma generator, or an ozonator for generating radicals and the system became complicated.
Therefore, the present invention has been accomplished under such circumstances and an object of the invention is to provide a film-forming method and a film-forming system capable of achieving satisfactory repeatability and uniformity of thickness and adequately large deposition rates in the film formation of a thin film on the substrate and simplifying the system configuration.
In order to solve the above problems, the inventors have been conducted intensive and extensive research and found a film-forming pattern demonstrating different behavior from the conventional dry oxidation or wet oxidation on the occasion of forming an oxide film on a silicon (Si) substrate. The inventors continued the research about the reaction mechanism of oxidation in this film formation, discovered that chemically active oxidizing factors (chemically active species) were involved in the reaction, and found preferred film-forming conditions, thus completing the present invention.
Specifically, a film-forming method according to the present invention is a method for forming a thin film on a substrate to be treated, which comprises a pressure reducing step of reducing pressure around the substrate, a heating step of heating the substrate, and a reactant gas supply step of supplying a first gas and a second gas, said second gas being capable of releasing energy by reaction with the first gas, so as to mix the first gas and the second gas, onto the substrate. These pressure reducing step, heating step, and reactant gas supply step do not have to be simultaneously started and stopped, but it is desirable to first carry out the pressure reducing step to reduce the pressure around the substrate and thereafter continuously perform the reactant gas supply step of supplying the gases onto the substrate to maintain the pressure of the reactant gases at a predetermined pressure, while heating the substrate.
In this film-forming method, the first and second gases supplied so as to mix onto the substrate can react with each other above the vicinity of the substrate by the heating of the substrate. This results in generating a variety of chemically active reaction species (chemically active species) immediately above the substrate, so that the surface of the substrate is exposed to these chemically active species. The chemically active species are considered to reach the interface (outermost surface) of the substrate and exert their energy on the constituents of the interface to promote reactions such as decomposition, dissociation, and the like of the constituents and reactions thereof with the chemically active species themselves.
Such reactions gradually proceed from the interface to the interior of the substrate, whereby a thin film of reaction products is formed in a predetermined thickness on the surface of the substrate. It was then verified that the reactivity of the film formation was able to be enhanced by carrying out this film-forming method under a reduced pressure condition.
The film-forming method is preferably one further comprising a substrate setting step of bringing the substrate into a chamber having a support section on which the substrate is set to be supported and a heating section opposed to the substrate and functioning to heat the substrate, and setting the substrate on the support section, wherein the pressure reducing step comprises a step of reducing the pressure around the substrate by reducing pressure in the chamber housing the substrate, wherein the heating step comprises a step of heating the substrate supported on the support section, by the heating section, and wherein the reactant gas supply step comprises a step of supplying the first gas and the second gas so as to mix the first and second gases, to between the heating section and the substrate supported on the support section. This makes it easier to maintain the reduced pressure state around the substrate, i.e., to maintain the first and second gases in predetermined concentrations. Since the first gas and the second gas exist between the substrate and the heating section, reaction efficiency between them is increased.
Further, it is desirable that in the pressure reducing step and the reactant gas supply step, the pressure around the substrate or the pressure in the chamber is regulated to 0.5 to 2 kPa (about 4 to 15 Torr). When the pressure inside the chamber is controlled in this range, the film-forming rate becomes satisfactorily high and an extremely thin film can be formed with excellent uniformity and repeatability of thickness.
Yet further, it is also useful that the reactant gas supply step comprises a step of varying a mixture ratio of the first gas and the second gas or a step of varying a feed of at least either one of the first gas and the second gas. Execution of the former step will result in varying concentrations and the composition of the chemically active species evolved from the reaction between the two gases, while the latter step is suitable for pressure control (concentration control) at a constant mixture ratio of the two gases and can substantially also serve as the former step.
Yet further, the reactant gas supply step is preferably a step using a gas having hydrogen atoms in molecules, as the first gas and using an oxygen gas as the second gas. In this case, the first gas is oxidized to bring about so-called combustion reaction and there is a tendency to facilitate attainment of high reaction energy. As a result, it is feasible to increase the concentrations and energy of the chemically active species evolved from the reaction between the two gases.
Specifically, the first gas is more preferably hydrogen gas. In this film-forming method, the mixture of hydrogen gas and oxygen gas burns (reacts) immediately near the heated substrate, for example, immediately above the substrate to evolve water (water vapor) through various elementary reactions, whereby the substrate is oxidized to form an oxide film. In this case, different from the conventional method for introducing the water (steam) generated by the external combustion, onto the substrate, the various chemically active species increase the percentage of their contribution on the oxidation reaction of the substrate.
Then reaction heat generated in the reaction between hydrogen gas and oxygen gas is imparted to chemical change such as dissociation or the like of the substances constituting the substrate, whereby the activation energy of the oxidation reaction of the substrate appears drastically reduced. For these reasons, the film-forming rates become extremely high. For example, it was verified with an Si wafer as the substrate that the film-forming rates of SiO2 film were drastically increased and sufficiently uniform thin films were able to be formed with satisfactory repeatability, as compared with the conventional dry oxidation processes.
More specifically, it is more preferable to use a substrate with a nitride film, as the substrate and form an oxide film as a thin film by oxidizing at least a part of the nitride film. For example, for using an Si wafer with a film of silicon nitride or the like as the substrate and oxidizing the silicon nitride film, the conventional wet oxidation by the external combustion method had the following problems;
1) the film-forming temperature had to be increased in order to raise the film-forming rates to a satisfactory level and, where there was restraint on the film-forming temperature, the film formation had to be carried out over a long period of time;
2) the quantity of input heat into the Si wafer became large, so that there was the possibility of causing change of shape such as warpage or the like of the Si wafer.
For forming a multilayered film, for example, of ONO (oxide-nitride-oxide) structure by the CVD (Chemical Vapor Deposition) method, particularly, by the high temperature CVD (HTO) method or the like, the layers were sequentially deposited. This means that three layer deposition steps had to be carried out in this case. At this time, the whole thin film consisting of the three layers must include dispersion of thickness of each layer. This dispersion of thickness raises the possibility of failing to adapt adequately for the film thinning demanded from the increase in the integration density of devices and there was thus the desire for further uniformity of thickness, particularly, for the Si wafers moving toward larger diameters.
In contrast with this conventional method, the present invention is such that, for example, for forming an oxide film on a nitride film formed by HTO, the nitride film itself is oxidized. Namely, the oxide film is not deposited on the nitride film. For forming the above ONO structure, an oxide film is further deposited by the conventional HTO method. Therefore, the thin film consisting of the three layers can be formed by carrying out only two deposition steps, so that the whole film includes only dispersion of thickness of the two layers made by the HTO method. thus the thickness uniformity of the thin film can be enhanced as compared with the conventional method. In addition, oxidation rates of the nitride film (film-forming rates of silicon oxide film) are drastically increased and the oxide film can be well formed by the treatment at lower temperature and in shorter time than by the conventional wet oxidation.
A fabrication method of a semiconductor device according to the present invention is a method for fabricating a semiconductor device comprising an interelectrode insulating film interposed between electrodes and having a nitride film and an oxide film, wherein at least a part of the oxide film in the interelectrode insulating film is formed by the film-forming method of the present invention. According to this fabrication method, at least a part of the nitride film on the substrate for construction of the semiconductor device is oxidized by the film-forming method of the present invention. This allows a thin oxide film to be formed with excellent uniformity of thickness and in desired thickness. As a result, the thin interlayer insulating film can be formed with excellent dielectric strength characteristic and in desired thickness.
Further, use of the aforementioned wet oxidation method and HTO method requires a thermal treatment at high temperature and over a long period, because oxidation rates are low. As a consequence, it may cause structural change of the electrodes previously formed, and this can negatively affect the device characteristics of the semiconductor device. In contrast with it, the fabrication method of the semiconductor device according to the present invention can fully suppress such negative effect, because the oxide film is formed by the thermal treatment at relatively low temperature and in relatively short time.
Further, the semiconductor device is preferably a nonvolatile memory comprising a stack of a floating gate electrode, a control gate electrode, and an interelectrode insulating film interposed between the floating gate electrode and the control gate electrode and having a nitride film and an oxide film; specifically, the fabrication method of the semiconductor device according to the present invention is preferably applied to fabrication of EEPROM (Electrically Erasable Programmable Read Only Memory). If a thermal treatment at high temperature and over long time were carried out during formation of the oxide film in the interelectrode insulating film of the nonvolatile (semiconductor) memory, it could deteriorate, for example, the tunnel oxide film on which the floating gate electrode of polysilicon structure is laid. This might result in degrading rewriting reliability being one important property of the nonvolatile memory.
Particularly, for the EEPROM (flash memory) with the electrically rewritable function, this degradation of rewriting reliability is a significant issue. In contrast with it, the fabrication method of the semiconductor device according to the present invention permits the oxide film to be fabricated by the thermal treatment at relatively low temperature and in relatively short time, and thus it can adequately suppress the degradation of the rewriting reliability of the nonvolatile memory.
A film-forming apparatus (system) according to the present invention is an apparatus (a system) for suitably carrying out the film-forming method of the present invention, which is a system for forming a thin film on a substrate to be treated. The film-forming system comprises a pressure reducing section for reducing pressure around the substrate, a heating section for heating the substrate, and a reactant gas supply section for supplying a first gas and a second gas, said second gas being capable of releasing energy by reaction with the first gas, so as to mix the first gas and the second gas, onto the substrate.
Further, the film-forming system is preferably one further comprising a chamber, said chamber having a support section on which the substrate is mounted to be supported, and the heating section opposed to the substrate, wherein the pressure reducing section is a section for reducing pressure inside the chamber and wherein the reactant gas supply section is a section for supplying the first gas and the second gas so as to mix the first gas and the second gas, to between the heating section and the substrate supported on the support section.
Yet further, the pressure reducing section is preferably a section capable of regulating the pressure around the substrate or the pressure inside the chamber to 0.5 to 2 kPa (about 4 to 15 Torr). Moreover, the reactant gas supply section preferably comprises a first flow control for controlling a feed of the first gas and a second flow control for controlling a feed of the second gas. In addition, the reactant gas supply section is more preferably a section for supplying, preferably, a gas having hydrogen atoms in molecules or, particularly preferably, hydrogen gas as the first gas and for supplying oxygen gas as the second gas.
It is more preferable that the substrate is a substrate with a nitride film formed thereon and that the film-forming system be one for forming an oxide film as a thin film by oxidizing at least a part of the nitride film.
A semiconductor device according to the present invention is one effectively fabricated by the fabrication method of the semiconductor device according to the present invention. Namely, the semiconductor device of the present invention is one comprising an interelectrode insulating film interposed between electrodes and having a nitride film and an oxide film, wherein at least a part of the oxide film in the interelectrode insulating film is formed by the film-forming system or the film-forming method according to the present invention.
Here the present invention acts extremely effective when the semiconductor device is a nonvolatile memory comprising a stack of a floating gate electrode, a control gate electrode, and an interelectrode insulating film interposed between the floating gate electrode and the control gate electrode and having a nitride film and an oxide film; particularly, an EEPROM.